The existence and use of ultra-hard materials in the form of polycrystalline material types for forming tooling, cutting and/or wear elements is well known in the art. Two polycrystalline material types, polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PcBN), are used for example as cutting elements to remove metals, rock, plastic and a variety of composite materials. Such known polycrystalline materials have a microstructure characterized by a polycrystalline matrix first phase, that generally occupies the highest volume percent in the microstructure and that has the greatest hardness, and a plurality of second phases, that are generally filled with a catalyst material that was used to bond or sinter the materials forming the polycrystalline matrix first phase.
For example, conventional PCD is formed by combining diamond grains (that will form the polycrystalline matrix first phase) with a suitable solvent catalyst material (that will form the second phase) to form a mixture. The mixture is subjected to processing conditions of extremely high pressure/high temperature, during which process the solvent catalyst material promotes desired intercrystalline diamond-to-diamond bonding between the grains, thereby forming a PCD structure.
Solvent catalyst materials typically used for forming conventional PCD include solvent metals from Group VIII of the Periodic table, with cobalt (Co) being the most common. Conventional PCD can comprise from about 85 to 95% by volume diamond and a remaining amount being the solvent metal catalyst material. The solvent metal catalyst material is present in the microstructure of the PCD material within interstices that exist between the bonded together diamond grains and/or along the surfaces of the diamond crystals.
The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired. Industries that utilize such polycrystalline materials for cutting, e.g., in the form of a cutting element, elements include automotive, oil and gas, aerospace, nuclear and transportation to mention only a few.
For use in the oil production industry, such PCD cutting elements are provided in the form of specially designed cutting elements such as shear cutters that are configured for attachment with a subterranean drilling device, e.g., a shear bit. Thus, such PCD shear cutters are used as the cutting elements in shear bits that drill holes in the earth for oil and gas exploration. Such shear cutters generally comprise a PCD body that is joined to substrate, e.g., a substrate that is formed from cemented tungsten carbide. The shear cutter is manufactured using an ultra high pressure/temperature process that generally utilizes cobalt as a catalytic second phase material that facilitates liquid phase sintering between diamond particles to form a single interconnected polycrystalline matrix of diamond with cobalt dispersed throughout the matrix.
The shear cutter is attached to the shear bit via the substrate, usually by a braze material, leaving the PCD body exposed as a cutting element to shear rock as the shear bit rotates. High forces are generated at the PCD/rock interface to shear the rock away. In addition, high temperatures are generated at this cutting interface, which shorten the cutting life of the PCD cutting edge. High temperatures incurred during operation cause the cobalt in the diamond matrix to thermally expand and even change phase (from BCC to FCC), which expansion is know to cause the diamond crystalline bonds within the microstructure to be broken at or near the cutting edge, thereby also operating to reduces the life of the PCD cutter. Also, in high temperature oxidizing cutting environments, the cobalt in the PCD matrix will facilitate the conversion of diamond back to graphite, which is also known to radically decrease the performance life of the cutting element. Additionally, the presence of cobalt in the polycrystalline matrix is also known to inhibit the ability of heat to be transferred away from the cutting edge of the cutter, because the coefficient of heat transfer for cobalt is approximately 100 W/mK as compared to diamond that is about 500-2600 W/mK.
Attempts have been made to address the above-noted limitations, associated with the presence of cobalt in the polycrystalline matrix, for the purpose of enhancing the service life of PCD cutting elements. For example, it is known to treat PCD to remove the cobalt or second phase material therefrom, which treatment has been shown to produce a resulting diamond body having enhanced cutting performance. One known for doing this involves at least a two-stage technique of first forming a conventional sintered PCD body, by combining diamond grains and a cobalt solvent catalyst material and subjecting the same to high pressure/high temperature process as described above, and then removing the solvent catalyst material therefrom, e.g., by acid leaching process.
However, the approach of removing the second phase cobalt from the polycrystalline diamond matrix creates the formation of voids or empty pores within the matrix surrounding the diamond crystals. The presence of such voids results in the formation of a porous structure that, while providing somewhat improved thermal expansion properties, now lacks strength and fracture toughness. In addition to the diamond structure lacking such physical properties, the presence of the voids within the microstructure act as insulation like empty air spaces, thereby impairing or reducing thermal heat transfer within the microstructure by relying on convective rather than conductive heat transfer within these voids.
It is, therefore, desirable that an ultra-hard construction be engineered in a manner that not only provides for improved properties of thermal stability, but that does so in a manner that does not sacrifice mechanical properties such as strength and fracture toughness. It is also desired that such ultra-hard constructions be engineered in a manner that provides improved thermal transfer characteristics when compared to conventional thermally stable PCD, by improving one or all of the thermal transfer mechanisms of conduction, convection and/or radiation within the material microstructure and/or construction.